Generally, liquid crystal has both the fluidity of liquid and the optical properties of crystal and so is classified as a material having intermediate properties between liquid and solid. Optical anisotropy of the liquid crystal can be changed by either an electric field or heat. Liquid crystal displays (LCDs) using these properties of liquid crystal are representative of flat panel display devices along with plasma displays and electroluminescent displays.
FIG. 1 illustrates phase transition process of the liquid crystal compound according to temperature and FIG. 2 illustrates directional propertiesof the liquid crystal molecules at each liquid crystalline phase, in detail. Liquid crystal molecules in nematic mesophase are aligned in one direction and are aligned along one dimensional molecular arrangement having no regularity in the center position of the molecule. Since intermolecular interaction between terminals of the molecules is larger than attraction between adjacent molecules, sliding is generated between neighboring molecules through shear force and each molecule is orientated along the molecular chain direction. Liquid crystal molecules in smectic mesophase are aligned in two-dimensions with each molecule aligned in one direction forming a layered structure in which the end portion of each molecule is uniformly arranged with respect to a surface, simultaneously. Accordingly, liquid crystal of smectic mesophase has a larger viscosity coefficient and lower fluidity than that of nematic mesophase. In cholesteric mesophase, the molecular arrangement in one plane has a nematic structure. However, the directional factor in each plane shows a spiral shape with respect to the vertical axis as the central axis. The pitch is changed by external force such as temperature and electric fields and the reflection state of light is also changed. Accordingly, the liquid crystal of cholesteric mesophase has the diverse color changing characteristics (multichroic characteristics) and hence receives attention from application field requiring diverse colors.
Most of the conventional cholesteric liquid crystal compounds have zero or one phenyl group in the center group as shown below and have low transition temperature and inferior stability. Therefore, the compounds are not applicable in display devices operating within wide temperature ranges and phase-changed in an undesired temperature range. ##STR1## wherein, R is C.sub.n H.sub.2n+1 and n is an integer of 1-22.